• M.Sc. Detrital Zircon Geochronology and Rift-related Magmatism: Central Mackenzie Mountains, Northwest Territories - Christopher Leslie and Dr. James K. Mortensen

• M.Sc. Structural studies along the Plateau Fault - Justin MacDonald and Dr. Shoufa Lin

• M.Sc. Controls on Zn-(Pb) mineralization in the Lower Cambrian Sekwi Formation, Mackenzie Mountains zinc district, Northwest Territories - Beth Fischer and Dr. Elizabeth C. Turner and Daniel J. Kontak

• Ph.D. Mid-Cretaceous granitoids in the southwestern Northwest Territories and southeastern Yukon: magma source regions, tectonic setting, and metallogeny - Kirsten L. Rasmussen and Dr. James K. Mortensen

• B.Sc. Geology and mineralogy of the Mountain River Beryl (variety emerald) Showing, Mackenzie Mountains, Northwest Territories – Melanie Mercier and Dr. Andre Lalonde
  

Detrital Zircon Geochronology and Rift-related Magmatism: Central Mackenzie Mountains, Northwest Territories

Neoproterozoic to Cretaceous strata in the Mackenzie Mountains of the northern Canadian Cordillera record many geological events that affected the western margin of Ancestral North America. Two of these events that are the focus of this study (Leslie 2009) are; (1) the development of a thick long-lived passive margin sequence of sedimentary rocks; and (2) continental rifting of this passive margin in Ordovician time that was accompanied with alkaline magmatism.

The first study documents a detrital zircon dating investigation of sedimentary strata from throughout the Neoproterozoic to Cretaceous sedimentary sequence in the Mackenzie Mountains. Speculations on the provenance of these sediments are based on these new U-Pb detrital zircon ages. Igneous and sedimentary sources comprising the North American craton are discussed and considered. These sources may have been exposed periodically and may not be entirely local to northwestern North America.

The second study consists of a comprehensive petrographical, geochemical, isotopic and geochronological study on alkaline igneous rocks associated with the development of a local rift basin termed the Misty Creek Embayment located in the central Mackenzie Mountains.

The Misty Creek Embayment records two Early Paleozoic rifting events; one in Cambrian time and the second in Ordovician time. The Ordovician event was responsible for the generation of widespread mafic alkaline volcanic and associated intrusive rocks. Speculations on igneous petrogenesis of these rocks including the nature and depth of the source, the degrees of partial melting, and the affects of magma generation on overlying lithosphere are discussed in this study.

This thesis can be downloaded:

• Leslie, C.D., 2009, Detrital zircon geochronology and rift-related magmatism: central Mackenzie Mountains, Northwest Territories [M.Sc. thesis]: University of British Columbia, Vancouver, BC, 236 p., http://hdl.handle.net/2429/7109

An investigation of the Ten Stone Ranges Structural Complex; Footwall to the Structural studies along the Plateau Fault

Justin MacDonald and Dr. Shoufa Lin
University of Waterloo

A structural study was completed as part of the Sekwi Mountain Project through Dr. Shoufa Lin at the University of Waterloo which involved 1:50 000 scale mapping within the NTS sheet 106A (Mount Eduni). The Mount Eduni map area contains a unique NW-SE striking zone, the Ten Stone Ranges Structural Complex, which is bounded on the SW by the Plateau Fault and on the NE by the Stony Range Anticline. This zone is the heart of the fold-and-thrust belt whereby numerous NE vergent dip-slip faults and décollement folds surface from one basal detachment. Determining the approximate depth of this detachment was completed by constructing a regional area-balanced cross section and using an “excess-area” depth to detachment calculation on the Stony Range Anticline, a regional box fold NE of the Ten Stone Ranges Structural Complex.

In addition to determining an approximate depth of the regional detachment, eight serial cross sections were constructed across the structural grain of the Ten Stone Ranges Structural Complex which were used to better understand the three dimensional geometry of a complex fault-bend-fold system, the Cache Lake Anticline.

This thesis is available from the University of Waterloo and can be viewed the Northwest Territories Geoscience Office library:

MacDonald, J.D., 2009: The Ten Stone Ranges Structural Complex of the central Mackenzie Mountains fold-and-thrust belt: A structural analysis with implications on the Plateau Fault and regional detachment level [M.Sc. thesis]: University of Waterloo, Waterloo, ON, 95 p.

	Figure 1. Simplified map of the study area (Ten Stone Range Structural Complex) in central NTS sheet 106A, showing location of cross sections.

	Figure 2: Regional cross-section restored to original state before deformation. Constructed using a combination of “Equal-Area” and “Key-Bed” balancing techniques (Mitra et al., 1989). Fault trajectories are dashed beyond the current erosional level.

Controls on Zn-(Pb) mineralization in the Lower Cambrian Sekwi Formation, Mackenzie Mountains zinc district, Northwest Territories

Beth J. Fischer and Elizabeth C. Turner and Dr. Daniel J. Kontak
Laurentian University

Carbonate-hosted Zn-Pb mineralization has been known in the Mackenzie Mountains for over 50 years. Regional exploration peaked in the mid-1970s, and saw a moderate recent resurgence prior to the economic downturn of 2008. Zinc occurrences are known from every carbonate-dominated stratigraphic interval in the district, from Neoproterozoic to Devonian. Age(s) of the mineralization is not known. Zinc and subordinate lead sulphide mineralization is known from over 30 locations along a 300 km trend of the Lower Cambrian Sekwi Formation (Fig. 1). The Sekwi Formation is a succession of carbonate and minor terrigenous clastic rocks deposited on a ramp and slope on the western passive margin of ancestral North America.

The objectives of this study were to elucidate first order controls on mineralization in the Sekwi Formation, and to constrain the nature of the fluids responsible for that mineralization. This was done by combining the results of property-scale mapping, detailed stratigraphy of mineralized successions, and petrography, with compilation and interpretation of previously published stratigraphic data; and by fluid inclusion microthermometry and isotopic studies on mineralized samples. Three mineralized areas lying along 130 km strike length of Sekwi Formation were examined. These were, from NW to SE, the AB, TIC, and Palm areas (Fig. 1). Mineralization is both replacive and void-filling with the latter predominating volumetrically (Fig. 2).

An NWT Open File report and map at 1:20,000 scale, describing the stratigraphy and structure of the AB area, will be published by NTGO in July 2009.

Preliminary conclusions of the work are:

• There is no stratigraphic control that localizes mineralization within the Sekwi Formation. Showings occur at various stratigraphic levels in the upper, the lower, and possibly the middle sandy members. Although this work was not extensive enough to say for certain, it appears there is no stratigraphic control throughout the Neoproterozoic to Devonian succession either.
• Fracturing and faulting at multiple scales, from hand specimen to regional, are invariably associated spatially with mineralization (Fig. 3), and represent a key control. Regional and property-scale faults, although barren, served as conduits, whereas micro- and meso-scale fractures localized precipitation in susceptible lithologies.
• Lithologies with millimetre- to centimetre-scale variations in mineralogy and therefore in alteration potential, common in the Sekwi Formation, are an important control on mineralization only in proximity to fluid conduits. Such lithologies include ooid-intraclast dolograinstone and dolorudstone, skeletal dolowackestone with peloidal dolograinstone matrix, fenestral dolostone, and burrowed dolostone to dolowackestone rich in siliciclastic silt and organic material.
• Dolomitization spatially associated with mineralization post-dated at least some burial diagenesis, and occurred syn- to post-mineralization. This suggests that mineralization is post-diagenetic.
• At least two fluids participated in sphalerite precipitation at AB and TIC. Weakly defined populations of cooler (150-175°C) and warmer (200-240°C) fluids have been identified, as well as dilute (2-8 wt%) and saline (12-20 wt%) fluids. A paucity of data points precludes identification of mixing trends.
• The temperature and salinity characteristics of Sekwi Formation fluids are very similar to those of the Silvermines and Tynagh deposits in the Irish district, somewhat less saline than but comparable in temperature to the Gays River deposit in Nova Scotia, and 10-160°C hotter than classic MVT deposits from the Mississippi Valley district.
• Sulphide sulphur from AB, TIC, and Palm came from an evaporite, likely the Gypsum formation of the Little Dal Group. It was reduced bacterially at Palm and thermochemically at TIC and AB. The geological reason for this difference is not apparent.
• 87Sr/86Sr ratios of sphalerite from 0.712 to 0.730 demonstrate that mineralizing fluids spent some time in a highly radiogenic reservoir. We suggest that fluids passed through felsic crystalline basement and rose along faults to sites of deposition in lithologically favourable strata.
• Higher strontium ratios from TIC and one of the many AB showings indicate either a longer duration of interaction of their fluids with the radiogenic reservoir (for example, by circulation through deeper basement faults), or passage of their fluids through different source rocks, richer in radiogenic strontium.

	Figure 2. Red and green sphalerite in a 3 cm vug at the TIC showing.

Mid-Cretaceous granitoids in the southwestern Northwest Territories and southeastern Yukon: magma source regions, tectonic setting, and metallogeny

Kirsten L. Rasmussen and Dr. Jim K. Mortensen
University of British Columbia

This PhD thesis aims to examine the petrogenesis of voluminous, early to late Cretaceous felsic magmatism and related mineralization emplaced into Laurentian and peri-Laurentian terranes in the Yukon and the Northwest Territories (Fig. 1). Ultimately, this study will allow us to: (1) understand in better detail the petrogenesis of the magmatism, the nature of basement rocks, and make inferences as to melt, metal, and ligand sources; and, (2) add to our current but limited understanding of the current tectonic, magmatic, and metallogenic model for the Cretaceous, and allow us to add more detail and possibly make modifications to the existing model. These objectives will be accomplished by:

1. filling gaps in the dataset for the Cretaceous granitoids by analyzing 65 samples for geochronological-geochemical-(radiogenic)isotopic data from over 50 intrusions.
2. using new analytical techniques to produce one of the largest published S±O isotopic datasets of plutons and related mineralization; and, application of LA-ICP-MS to document inheritance and to determine Lu-Hf isotopic compositions of zircon.
3. examining the evolution of magmatic volatiles (F-Cl-SO3) in intrusive phases on a regional basis, and locally in tungsten mineralizing magmatic systems.
4. compiling all the available relevant datasets (e.g., Yukon Igneous Database; published and unpublished data sources) and combining them with the new data from this study in order to examine and test the existing model for the tectonic, magmatic, and metallogenic evolution of the northern Cordillera during the Cretaceous.

Completion of this study will be September 2010. Research topics include, and products will include the following:

• Geochronological, Geochemical, and Radiogenic Isotopic Characterization of Magmatism in a Lower Plate and an Upper Plate Rifted Margin
• Stable Isotopic Compositions of Cretaceous Magmatism in the Northern Cordillera: basement composition and sulfur sources
• Magmatic Fluid Compositions (F-Cl-SO3) in the Intrusion-related Tungsten Mineralizing Environment
• A Model for the Tectonic, Magmatic, and Metallogenic Evolution of the Northern Cordillera in the Cretaceous: implications for other large felsic magmatic provinces

Geology and mineralogy of the Mountain River Beryl (variety emerald) Showing, Mackenzie Mountains, Northwest Territories

Melanie Mercier and Dr. Andre Lalonde

The Mountain River beryl showing (MRBS) was discovered in 2007 during regional mapping the Mackenzie Mountains, Northwest Territories, by the Sekwi Mountain project. The showing comprises several small outcrops on a steep hillside south-southeast of Shale (Palmer) Lake, where beryl is found in numerous quartz-andesine-dolomite veins with minor disseminated pyrite. The veins are hosted in thinly-laminated to medium-bedded pyritic sandstone with fine interbeds of siltsone of the Neoproterozoic Twitya Formation. Powder X-ray diffraction and electron microprobe analyses confirm that the mineral is beryl. The MRBS is the first beryl occurrence to be documented in the region.

Beryl occurs as hexagonal prismatic crystals that are 1 to 5 millimetres in diameter and up to 4 centimetres in length. The crystals are vitreous, translucent, and commonly disposed in radiating clusters or grow perpendicular to the vein margins. They are brilliant green in colour, which corresponds to the 5G 6/6 reference of the Munsell rock-colour chart. The arrangement of the crystals and the vuggy nature of the veins suggest that the veins are extensional fracture fillings. Trace elements analyses of the crystals reveal elevated amounts of V and Cr, both recognized for causing green colour in minerals, along with Fe and Sc. The green colour of the beryl along with its specific trace element concentrations indicates that the variety is emerald.

	Figure 1. Radiating clusters of beryl (variety emerald) at the Mountain River Beryl Showing, NTS sheet 106, Mackenzie Mountains, Northwest Territories.

The source of the hydrothermal fluids responsible for the veining and associated beryl and the age constraint of the veins at this locality are uncertain. The closest known Cretaceous intrusions, responsible for other emeralds occurences in the northern Cordillera, are located 80 kilometers to the south and the Silurian alkali Mountain diatreme is 14 kilometres to the west. The lack of evidence of igneous activity or metamorphism suggests that the fluids were channeled along thrust faults that are present in the area.

This thesis is available from the University of Ottawa and can be viewed the Northwest Territories Geoscience Office library:

• Melanie Mercier. 2008. Geology and mineralogy of the Mountain River Beryl (variety emerald) Showing, Mackenzie Mountains, Northwest Territories. B.Sc. thesis, Department of Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada.